Petrologic Evolution of a Nipigon Diabase Sill, Ontario, Canada: Insights From Compositional and Textural Profiles

Abstract

Petrographic and geochemical profiles collected through a ~250-m-thick Nipigon diabase sill near Black Sturgeon Lake, Ontario, constrain the complex emplacement and differentiation processes that controlled the magmatic evolution of these rocks: several nested reinjections of phenocryst-bearing magma, compactiondriven redistribution of mildly evolved interstitial liquids in the central portion of the sill, and segregation of highly-evolved interstitial liquids into micropegmatitic veins near the upper contact. These processes impact not only the silicate mineralogy of the rocks, but also distribution of sulfide phases. Both petrographic and geochemical data demonstrate that the sill was gradually inflated via several discrete injections of olivine-bearing magma. Plagioclase crystal size trends exhibit several reversals that are correlated with increases in olivine abundance and bulk-rock MgO content. These textural and compositional reversals are attributed to reinjections of olivine-bearing magma into partially solidified magma from earlier injections. Mixing between evolved resident magma and reinjections of less-evolved magma is commonly proposed as a mechanism for triggering sulfide mineralization. In this sill, which contains no significant mineralization, localized minor enrichments and depletions in copper content are associated with the margins of a well-constrained reinjection horizon. Chemical differentiation of the magma was controlled by three distinct processes: flow differentiation, crystal-mush compaction, and solidification front instability. Although the mineralogy of the sill is dominated by plagioclase and clinopyroxene, major-oxide compositional variations are largely controlled by the mechanical concentration of olivine via flow differentiation or phenocryst redistribution within the individual reinjection pulses. This redistribution of phenocryst phases is the primary factor controlling the major-element composition of the rocks in most of the sill. In the central portion of the sill, the abundances of incompatible trace elements are influenced by the compaction of partially crystalline mush and the expulsion of incompatible-rich interstitial liquid. This is recognized by a negative correlation between the alignment of plagioclase crystals and concentrations of incompatible trace elements such as Zr. Finally, there is a ~2.5-m-thick silicic segregation approximately 32 m below the upper contact, whose composition is consistent with residual liquid after ~65 to 85% fractional crystallization of magma with a composition equivalent to that of the marginal rocks. Similar veins and patches are common throughout the upper 50 m of the sill, indicating that this segregation process was common and widespread during the cooling and crystallization of the sill. The complexity of intrusion and differentiation in this sill, despite its generally unremarkable texture and composition, suggests that many diabase sills may have experienced a range of processes that have previously been recognized to impact the petrologic evolution of larger igneous systems, and that these processes can be identified in minor intrusions such as sills and dikes using simple and robust analytical techniques.